This is one of the most common problem in Pinta. It should be fixed in Pinta 1.7-development version which is currently very stable (in my humble opinion more then official "Pinta 1.6 stable"). See bug report in you are interested in details: https://bugs.launchpad.net/pinta/+bug/1425749

WORKAROUND: If using Windows then download latest "Windows" file from https://pinta-project.com/pintaproject/pinta/builds currently latest successfully build version is 1.7.0.232. Then install it as production version, no particular difference in installation process.

If using Ubuntu you can use pinta-daily!!! PPA as documented: https://pinta-project.com/pintaproject/pinta/howto/installing-pinta

If using Linux in general (also Ubuntu) you can download "zip" file from https://pinta-project.com/pintaproject/pinta/builds latest successfully build version is 1.7.0.232. Extract zip file in some directory you like. Then in terminal "cd pinta" to move to directory and then execute "mono Pinta.exe" and Pinta will start-up. Check the Help | About version it should be 1.7.0.232, this is development version. With this process you can't broke existing Linux system and you can't broke existing installed stable Pinta version. If you don't like this pinta development version, then just delete the folder.

If using OSX, then process should be the same as on Linux, but I am not 100% sure, because I don't have OSX system to test this out.

Please report back if this problem is solved.